Method of separating tetravalent plutonium values from cerium sub-group rare earth values



thousands of years.

METHOD OF SEPARATING TETRAVALENT PLU- TONIUM VALUES FROM CERIUM SUB-GROUP RARE EARTH VALUES No Drawing. ApplicationiMa'rchIZ, 1947 Serial N0. 734,218

3' Claims. (Cl. 2314.5)

This invention is concerned with an improved method of separating plutonium from certain contaminating elements.

The word plutonium7 as. hereinafter used in the specification and claims refers to the element with atomic number .94 and to the compounds thereof, unless the context indicates clearly that plutonium is referred to in its elemental or metallic state.

Plutonium is a transuranic element of .atomic number of 94 which has recentlybeen discovered. Plutonium is normally produced by the bombardment of a uranium mass with neutrons in a chain neutronic reactor of the pile type. The uranium used may bethe natura-luranium which consists mainly of U 4 as much U and .a very small percentage of "U or the uranium may be enriched by increasing the amount of U present. Fission of the U atoms present. liberate neutrons which in turn react with the U atoms present to form U The U is radioactive with a half-life of 23 v minutes and decays with beta-emission to form -Np which is radioactive with a half-life of 2.3 .days and decays with beta-emission to form P11 The fission of the U atom .is initiated by the reaction of the atom with a neutron. The fission is predominantly binary and may be exemplified with the following type of equation:

The products formed by theifission of the U atom are generally very unstable and highly radioactive. Such products usually undergo beta particle disintegration in undergoing fission is roughly-equivalent to the numberundergoing reaction to'form transuranic elements. ,Since the fission products are at least twice the number of nuclei undergoing fission, aged material from a chain reaction will contain approximately twice as many fission products atoms 'as plutonium atoms.

The various decay products or the initial fission frag- TIIGH'ES are referred to herein jas fission products. "These fission products fall into two general groups; alightgroup of atomic numbers from approximately 35 to 44 and a heavy group having atomic numbers ranging from about 51 to about 61. The various radioactive fission products have half-lives ranging from afraction of a second to Those having very shorthalfllives may be eliminated by aging the material for a,reasonable period before handling. Incidentally, this aging also serves to convert the Np present to plutonium. Those fission products with very long half-lives do not have sufiici'ently intense radiation to endanger personnel protected with moderate shielding. On the other hand, the fission products having half-lives ranging from a few days to a few years have dangerously intense radiations United States Patent" 0 "ice which cannot be .eliminatedby aging for practical storage periods. Lanthanum, cerium, and praseodymium of the cerium sub-group of rare earths are included in the group of fission products which are extremely troublesome in the neutronic mass because of their medium half-life.

The processing of plutonium to obtain a purer product is ordinarily broken down into three steps: 1) the extraction step which is concerned with the separation of the plutonium from the unreacted uranium which constitutes the bulk of the mass removed from the neutronic reactor, (2) :the decontamination step which isconcerned with the separation of :plutonium from the radioactive fission products which contaminate the mass, and (3;) the concentration step which :is concerned with obtaining the pure plutonium in a concentrated state.

One method of recovering plutonium from compositions containing contaminating elements is to form an aqueous solution from which plutonium may be precipitated, either directly or by carrier, as an insoluble plutonium compound. Arecovery of plutonium as an insoluble precipitate is particularly applicable to solutions in which any contaminating cations do not form insoluble compounds with the anion to be employed to precipitate the plutonium. Conversely, the separation of plutonium from fission products may also be effected by forming an insoluble precipitate of fission products in a solution containing plutonium in 'a soluble state and thus separating-fission products from the 'plutonium'leav- 'ing plutonium in a relatively pure state in the solution. By proper choice of anions and repeated precipitations plutonium of a high. degree of purity may be recovered by this method.

It is an object of this invention to provide a'novel method of separating plutonium from contaminants comprising elements of thecerium sub-group of rare earths.

It is an additional object of this invention to provide a novel method of recovering plutonium from aqueous solutions containing plutonium and members of the cerium sub-group of rare earth metals.

Additional objects will be made apparent from the following description.

It is generally known that the cerium subgroup of rare earth metals forms carbonates which are insoluble in neutral and alkaline solutions. We have discovered that'the plutonium in the +4 oxidation state will form plutonium carbonate which is a light tan'fiocculent precipitate. The precipitate is relatively insoluble and experiments have shown that its solubility in at room temperature is-0.0l gramperliter. The precipitate of plutonium carbonate is formed in tan-approximately neutral solution with approximately ,stoichiometric amounts vof carbonate ion. Thus, the plutonium ca-rbonate may be formed by the introduction bicarbonate ions .into a solution containing the Po ion :in which the acidity of thesolution=istmaintained within thepH range .of approximately 5.5 to 7.5. Thecarbonateion may be introdueed'into the solution; by-the addition'of a suitable .solu'ble carbonatecompound such assodiurn or potassium carbonate. It may also be'desirable to use ,soluble bicarbonate salt but if a bicarbonate salt is used theacidity must then. be readjusted .to bring it within the neutral range. V

We have also discovered however, that the plutonium ion in the +4 state may be complexed in an aqueous solution so that the plutonium ion will not form an insoluble carbonate under conditions in which the cerium group of rare earths Will form insoluble carbonates. Broadly, the process of our invention comprises the method of separating plutonium from the cerium group in an aqueous inorganic acid solution. If the plutonium is present in the solution in an oxidation state greater than +4 plutonium should be reduced by any standard procedure so that all ions are present in a valence state of +4 or less. A precipitate of the cerium rare earth group carbonates is then formed by the addition of an excess carbonate ion. The carbonate ion may be added to the solution as the soluble carbonate salt of the alkali metals. It is preferable to use the sodium or potassium carbonate. The ammonium carbonate should be avoided since the ammonium carbonate has a tendency to complex the cerium rare earth carbonates as well as the plutonium. The bicarbonates of sodium and potassium have also been found to be suitable sources of carbonate ions. Both the carbonates of the cerium sub-group of rare earths, and plutonium are soluble in strongly acid solutions, so that in order to form a carbonate precipitate in the solution,

-it is necessary that the solution be basic or not more than slightly acid, and it has been found that these carbonates will precipitate in solutions with a pH greater than about 5.5. The acidity may, of course, be adjusted by the carbonate or bicarbonate salt added to the solution. In order, however, for the carbonate ion to complex the Pu+ ion, it is necessary that carbonate ion be present in excess. Although in some cases a separation of Pu+ ions and the cerium group of rare earths may be effected where a carbonate precipitate is formed in a solution in which the carbonate ions are present in solution'in less than 1 M concentration. it is desirable that the concentration of the carbonate ion be at least 1 M in order that the maximum separation be achieved.

The exact method by which the Pu+ ions are complexed by excess carbonate ions is not definitely known and we do not desire to be bound by any certain theory in this disclosure, but it is believed that the Pu+ ions may form an insoluble carbonate which is then re-dissolved in the excess carbonate ion. In any event where the excess carbonate ion is introduced into the solution containing the plutonium ions and cerium rare earth ions, a precipitate comprising substantially all of the cerium group of rare earths is formed in, .and may be separated from the solution leaving the plutonium ions in the supernatant liquid in substantially quantitative proportions. The rare earth carbonates which are formed may be readily separated from the solution by any of the usual methods such as centrifugation, filtration or decantation.

Following the separating of the cerium group of rare earth carbonates from the solution the plutonium may be precipitated by any standard method. One great advantage of this process of separation is that the excess anion may be removed from the solution containing the plutonium merely by heating and it will be seen that this is a great advantage over other methods of separation which often complete the separation leaving highly corrosive anions in solution, or leaving anions which are difficult to remove from the solution quantitatively by simple means.

This embodiment of our invention may be illustrated by the following examples:

'found to be preferable.

' 4 Example I About .3 mg. of La ion in the lanthanum nitrate form was added to an acid solution containing plutonium in its +4 oxidation state. The solution was made 1 M in Na CO andthe precipitate which was formed upon the addition of the sodium carbonate was removed from the solution bycentrifugation. An analysis of the precipitate of lanthanum carbonate by counter technique disclosed that the precipitate contained less than 1% of the plutonium. 4

In a similar experiment which varied from the above described example only in that K CO was used instead of Na CO only 3% of the 94 was carried from the solution with the precipitate of lanthanum carbonate.

Although an excess carbonate ion is used as the complex'ing agent for Pa in a preferred embodiment of our invention, we have discovered that other complexing agents may also be used and insome cases have been Thus, one advantageous complexing agent has been found to be the uranyl ion. The complexing of Pu+ ion by the uranyl ion is demonstrated by the following experiment:

A solution containing Pu+ ion was formed and lanthanum nitrate was added to the solution to furnish lanthanum ions. The solution was then made 0.25 M

inNaI-ICO and the precipitate which formed was separated. Counteranalysis disclosed that the precipitate had carried 81% of the plutonium. The experiment was then repeated under identical conditions except that 300 mg. of uranyl nitrate was added to the solution. The precipitate obtained under these conditions was found to contain less than 5% of the plutonium.

While there have been described certain embodiments of our invention, it is to be understood that it is capable of many modifications. Changes, therefore, may be made without departing from the spirit and scope of the invention as described in the appended claims in which it is the intention to claim all novelties in the invention as broadly as possible in view of the prior art.

What is claimed is:

' 1. A method of separating tetravalent plutonium values from rare earth values of the cerium sub-group contained in an aqueous solution, comprising adding to said solution a complexing agent for plutonium selected from the group consisting of alkali metal carbonate and uranyl nitrate, adding alkali metal carbonate as a precipitating agent in a quantity to obtain a pH value of at least 5.5 so that 'the rare earth values precipitate while the plutonium values remain in solution, and separating the precipitate formed from the solution.

2 The process of claim 1 wherein the alkali metal carbonate is added as the complexing agent in a quantity to yield a concentration of at least 1 M.

3. A method of separating tetravalent plutonium values from rare earth values of the cerium sub-group contained in an aqueous solution, comprising adding uranyl nitrate to said solution whereby the plutonium values are complexed, adding alkali metal carbonate to the solution in a quantity to yield a concentration of at least 1 M to precipitate said rare earth values but to retain the plutonium in solution, and separating the precipitate formed from said solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,767,044 Hill et al.v Oct. 16, 1956 2,768,871 Brown et al. Oct. 30, 1956 2,785,951 Thompson et al. Mar. 19, 1957 

1. A METHOD OF SEPARATING TETRAVALENT PLUTONIUM VALUES FROM RARE EARTH VALUES OF THE CERIUM SUB-GROUP CONTAINED IN AN AQUEOUS SOLUTION, COMPRISING ADDING TO SAID SOLUTION A COMPLEXING AGENT FOR PLUTONIUM SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL CARBONATE AND URANYL NITRATE, ADDING ALKALI METAL CARBONATE AS A PRECIPITATING AGENT IN A QUANTITY TO OBTAIN A PH VALUE OF AT LEAST 5.5 SO THAT THE RARE EARTH VALUES PRECIPITATE WHILE THE PLUTONIUM VALUES REMAIN IN SOLUTION, AND SPARATING THE PRECIPITATE FORMED FROM THE SOLUTION. 